Conventional base oils of gas oils are known to be produced by subjecting a straight-run gas oil or straight-run kerosene obtained from an atmospheric distillation unit for crude oil to a hydrorefining or hydrodesulfurization treatment. Conventional gas oil compositions are produced by blending one or more types of these base gas oils and base kerosenes. If necessary, these gas oil compositions are blended with additives such as a cetane number improver or a detergent (see, for example, Non-Patent Literature 1).
Furthermore, in recent years, due to changes in the structure of petroleum fuel demands, gas oil fractions obtained from a fluid catalytic cracker, thermal cracker or hydrocarcker, which fractions have been used mainly as base oils for heavy oil have been expected to be redundant, and it has thus been studied to hydrodesulfurize such gas oil fractions available from these units to be used effectively as base gas oils.
However, the base gas oils produced in this manner are poor in oxidation stability and as the result has been concerned for the formation of sludge or the formation of deposit during the engine combustion.
Therefore, it is necessary to produce a gas oil composition though containing such a base gas oil with a poor oxidation stability but capable of restraining the formation of sludge or deposit, reducing CO2 emission, improving the fuel economy and giving good influences on parts in an engine. Furthermore, since these engine-related performances are closely related to other fuel performances, it is extremely difficult to design a high quality fuel that can simultaneously achieve these requisite performances at higher levels.